In many discrete multi-tone (DMT) systems, a pilot sequence, consisting, for example, of amplitude and phase information, is used for timing recovery. Typically, the pilot sequence is transmitted at a fixed location (e.g., fixed frequency). If the transmission system includes channels that could have different frequency responses with unknown notches, using a fixed location for the pilot sequence could be problematic. For instance, the frequency of the pilot sequence could correspond to a deep notch in the channel frequency response, causing the pilot sequence to become significantly distorted prior to reception.
One attempt to solving the above problem is to use more than one tone for transmitting the pilot sequence. This approach, however, reduces the maximum possible data throughput in the system and continues to run the risk that all frequencies of the pilot sequences continue to correspond to channel notches.
Therefore, there exists a need for improved techniques for transmitting pilot sequences.